1. Field of the Invention
This invention relates generally to the field of seismic exploration and more particularly to an improved method and apparatus for imparting vibrational energy to the earth wherein selected combinations of compressional and shear wave energy can be generated simultaneously.
2. Description of the Prior Art
Recent advances in the ability of seismic data acquisition and processing equipment to handle large amounts of data have stimulated greater interest in the development of versatile high energy seismic sources adapted to produce both compressional and shear waves. One goal in the development of "multi-component" seismic sources is to enable the operator to more efficiently measure sub-surface formation properties such as porosity, fracture orientation, pore fluid type, or lithologic type. It is known that P, SH and SV waves travel with their own distinct velocities when passing through a particular geologic formation. Thus, changes in formation property such as rock type, porosity and the like may be detectable by changes in the full wave field (i.e., the P, SV and SH wave velocities). A strong correlation observed between lateral variations in the compressional to shear wave velocity ratio (V.sub.p /V.sub.s) with known changes in the geologic formation suggests that these variations may be used to infer changes in the lithology, porosity, or pore fluid content of a sub-surface formation. By recording the full wave form of a seismic wave, including P, SH and SV waves, one can get far more detailed information concerning rock properties at depth than would otherwise be possible.
There are two major types of surface seismic sources currently available, which have been adapted to the production of combined P and S waves. One is the impulsive type source wherein a heavy mass is propelled in a single downward stroke to impact a ground engaging target. The other is the reciprocating vibratory type transducer which imparts a controlled, varying frequency signal into the earth, as exemplified by the Vibroseis.RTM., (a registered trademark of Conoco, Inc.). It is with the latter type of seismic source that the invention to be described is particularly concerned.
The general concept of simultaneously imparting both compressional and shear wave vibrational energy to the earth with seismic exploration apparatus is taught in U.S. Pat. No. 4,321,981 issued to K. H. Waters on Mar. 30, 1982, entitled "Combination Shear Wave and Compressional Wave Seismic Energy Vibrator". In accordance with the above referenced patent, a combination of two or more vibrating masses may be operated along opposing non-vertical vibrational axes, oriented at a fixed angle to each other in parallel planes. The times of passage of the masses through a common central point as they vibrate along their respective axes can be controlled to establish a phase relation between the two vibratory motions. By varying the related phases of the two masses, one can effect desired inputs of compressional and shear waves simultaneously.
The referenced Waters patent teaches that the parallel planes in which the respective vibration axes of the two masses lie are to be oriented either transversely to the line of seismic survey, i.e., the line between the location of the seismic source and the recording geophones, or along such survey line. With proper phasing, the above referenced transverse axial orientation of the vibrating masses enables one to produce and record simultaneously P waves and shear SH waves. On the other hand, if the vibrational axes are aligned with the survey line, one can generate and record simultaneously P waves and shear SV waves. The vertical component of motion in both cases produces so called "Converted" shear SV waves but such waves cannot be processed aned interpreted with conventional seismic methods. Thus, Waters teaches in effect that only by operating the patented apparatus successively in these two modes can one generate and record at a single remote location all forms of a seismic wave, including P, SH and SV waves, in a manner which permits conventional processing.
The present invention has as its general objective, the teaching of a method and apparatus which is adapted to provide the generation of simultaneous P, SH and SV waves by means of vibrational energy.
In applicant's commonly assigned co-pending U.S. patent application, Ser. No. 772,878, filed Apr. 12, 1985 entitled, "Apparatus for Generating Combined Shear and Compressional Waves in the Earth", which is a continuation in part of application, Ser. No. 504,557, filed June 15, 1983, there is disclosed generally the method of simultaneous recording of the full wave form of a seismic signal, including P, SH and SV waves. The method involves the concept of impacting the earth with a force directed along a slant path inclined at a preselected acute angle with respect to a vertical direction and at an azimuth making a preselected acute angle with respect to a selected seismic line. This force may be resolved into three components directed respectively in a vertical path, and each of two horizontal paths, one being transverse to a given seismic line and the other being directed along such line. Thus, a single force against the earth delivered by a mass accelerated against a ground contacting target, may simultaneously generate P, SH and SV waves, which can be efficiently recorded by means of three orthogonal geophones at a remote location along the given seismic line.
Improved apparatus for directing the force of a surface seismic source along a slant path, such as described in the aforementioned application, Ser. No. 772,878, is set forth in Applicant's further co-pending commonly assigned U.S. patent application, Ser. No. 676762, filed Nov. 3, 1984; and entitled "Mounting and Control Means for Full Wave form Seismic Source". The pertinence of the teaching of the latter application to the present invention lies in the fact that it provides for free rotation of the impacting force path about two mutually perpendicular horizontal axes. Applicant has perceived that this "gimbal" type of support is well suited to the construction of a vibratory motion surface seismic source, with an adjustable vibratory axis.
It is therefore a more particular object of this invention to provide an improved method and apparatus for the simultaneous generation of P, SH and SV waves by means of the vibrational energy of a single vibrating mass.